1. Field of the Invention
The present invention relates to a device for measuring the period of a synchronizing signal in an input image signal, in a display device required to reflect a computer image, for example, which is suitable for a case where one of dots composing dot data of the input image signal and one pixel in the display device must be synchronized with each other on the basis of the period of the synchronizing signal, and a display device comprising the period measuring device.
2. Description of the Prior Art
A graphic signal produced by a computer is produced in a digital manner in units of dots (pixels) using a reference clock in the computer. However, the horizontal and vertical frequencies of the outputted image signal greatly differ depending on the maker of the computer and or the resolution of the image signal.
In a liquid crystal projector, for example, which is equipment for outputting a computer image, digital processing must be performed in complete synchronization with the dots (pixels) composing an inputted image signal. Therefore, the liquid crystal projector previously stores, for each of the frequencies of synchronizing signals in various types of image signals, data such as horizontal resolution, vertical resolution, the total number of dots in the horizontal direction, and the total number of lines in the vertical direction which are suitable for the image signal as a table.
The frequency of the synchronizing signal inputted simultaneously with the input image signal is measured, the data such as horizontal resolution, vertical resolution, the total number of dots in the horizontal direction, and the total number of lines in the vertical direction which correspond to the image signal whose frequency almost coincides with the frequency are read out of the table, and one of dots composing dot data of the input image signal and one pixel in a liquid crystal panel are synchronized with each other on the basis of the data read out, to perform digital processing such as scan conversion or filtering. Consequently, the frequency of the synchronizing signal inputted simultaneously with the input image signal must be accurately measured.
FIG. 6 illustrates the circuit configuration of a conventional synchronizing signal input section.
The conventional synchronizing signal input section is constituted by an RC filter 1 for noise reduction and a logic circuit 2 with a Schmidt trigger function. The characteristics of a cable for inputting an image signal and a synchronizing signal which are outputted form a computer into a projector cause ringing to occur in an edge of the synchronizing signal, as shown in FIG. 5.
An attempt to remove such ringing by the RC filter 1 makes it necessary to reduce the cut-off frequency of the RC filter. If the cut-off frequency of the RC filter is made too low, however, the synchronizing signal becomes dull, thereby delaying the synchronizing signal from the image signal as well as adversely affecting a synchronizing circuit in the succeeding stage.
The logic circuit 2 with a Schmidt trigger function greatly differs in the way of superimposition of noise by the characteristics of an input stage. This logic circuit 2 is effective for noise having a small amplitude when it is composed of a CMOS (Complementary Metal-Oxide Semiconductor), for example, while being unfavorable for a case where the amplitude of the synchronizing signal itself fed from the computer is small.
When the logic circuit 2 is composed of a TTL (Transistor-Transistor Logic), the logic circuit 2 is favorable for a case where the amplitude of the synchronizing signal itself is small and noise from the high potential side, while being unfavorable for noise having a small amplitude.
In the conventional synchronizing signal input section, measures to prevent noise in the synchronizing signal are thus insufficient. Accordingly, the frequency of the synchronizing signal cannot be accurately measured.
An object of the present invention is to provide a device for measuring the period of a synchronizing signal so adapted that the period of the synchronizing signal can be accurately measured even when ringing and noise are superimposed on the synchronizing signal, and a display device comprising the period measuring device.
A first period measuring device for measuring the period of a synchronizing signal in an input image signal according to the present invention is characterized by comprising a clamping circuit to which the synchronizing signal in the input image signal is inputted; a comparator for removing noise in the synchronizing signal outputted from the clamping circuit; period measuring means for measuring the period of the synchronizing signal on the basis of the synchronizing signal obtained by the comparator; judging means for judging whether or not the period of the synchronizing signal which has been measured by the period measuring means is stable; and means for controlling, when the judging means judges that the period of the synchronizing signal is not stable, a threshold voltage of the comparator such that the period of the synchronizing signal is stabilized.
A second period measuring device for measuring the period of a synchronizing signal in an input image signal according to the present invention is characterized by comprising a clamping circuit to which the synchronizing signal in the input image signal is inputted; a comparator for removing noise in the synchronizing signal outputted from the clamping circuit; a period measuring circuit for measuring the period of the synchronizing signal on the basis of the synchronizing signal obtained by the comparator; a judging circuit for judging whether or not the period of the synchronizing signal which has been measured by the period measuring circuit is stable; and a control circuit for controlling, when the judging means judges that the period of the synchronizing signal is not stable, a threshold voltage of the comparator such that the period of the synchronizing signal is stabilized.
A first display device according to the present invention is a display device comprising a period measuring device for measuring the period of a synchronizing signal in an input image signal, charaterized in that the period measuring device comprises a clamping circuit to which the synchronizing signal in the input image signal is inputted, a comparator for removing noise in the synchronizing signal outputted from the clamping circuit, period measuring means for measuring the period of the synchronizing signal on the basis of the synchronizing signal obtained by the comparator, judging means for judging whether or not the period of the sychronizing signal which has been measured by the period measuring means is stable, and means for controlling, when the judging means judges that the period of the synchronizing signal is not stable, a threshold voltage of the comparator such that the period of the sychronizing signal is stabilized.
A second display device according to the present invention is a display device comprising a period measuring device for measuring the period of a synchronizing signal in an input image signal, characterized in that the period measuring device comprises a clamping circuit to which the synchronizing signal in the input image signal is inputted, a comparator for removing noise in the synchronizing signal outputted from the clamping circuit, a period measuring circuit for measuring the period of the synchronizing signal on the basis of the synchronizing signal obtained by the comparator, a judging circuit for judging whether or not the period of the synchronizing signal which has been measured by the period measuring means is stable, and a control circuit for controlling, when the judging means judges that the period of the synchronizing signal is not stable, a threshold voltage of the comparator such that the period of the synchronizing signal is stabilized.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.